i'd probably have to say darth vader. his use of the force to control people simply puts him at the top of the list. also, he conquered most of the star wars galaxy, except for planets on the outskirts(i.e. Tatooine)
i know someone is gunna reply and say Darth Vader wasnt a general, but actually a sith lord. well actually, he was a general during the clone wars, first for the republic, then later for the empire.